The idea of nanopore based DNA sequencing has emerged from the need to diminish
the cost and increase the speed of genome sequencing. Traditional marker based
DNA sequencing needs much more time and also quite costly as all the biomolecules
targeted for sequential determination has to undergo fluorescent labeling [1]}, [2]}, [3]}.
Dramatic advancements in the DNA sequencing techniques in the last few years have huge
impact on genome research [4]}, [5]}, [6]}, [7]}, [8]}. As the understanding of genetic
components of different diseases now become more vivid, it leads to the development of new
type of medical treatments and diagnostics. Genetic research has many other applications
apart from human medicine, such as in security, biology and agriculture. But the main problem with
the current sequencing technology e.g., the well-established Sanger method [9]},
is its cost. Sequencing a specific human genome to high quality currently costs around
$10K depending on the procedures. Whereas the nanopore based DNA sequencing offers
marker-free single-molecule detection without any sample modification at much lower cost ($1000).
The initial attempts of nanopore based sequencing of single-stranded DNA (ss-DNA)
using protein pore (\(\alpha \) -Haemolysin) were made more than a decade ago by translocating
ss-DNA through the nanopore under an applied bias while reading the changes in the ionic
current passing through the pore [10]}, [11]}, [12]}, [13]}. Detection of the
signatures of the nucleotides trapped inside a nanopore i.e., of a static strand
in nanopore is easy [14]}, whereas the problem of detecting the responses of
the nucleotides of a translocating DNA through nanopore is still
a big challenge, as the speed of translocation is too high for getting the necessary
current resolution unless they are slowed down by chemical modifications with bulky
groups [15]}. The early usage of biological nanopore in DNA sequencing is mainly
due to the key factors like nanometer dimension of the pore, detectable ion-current
modification, and abundance of different processing enzymes and chemicals to regulate
the speed of DNA translocation in discrete steps [16]}, [17]}. However there
are some serious disadvantages with biological nanopores due to their poor mechanical
strength [18]}, and difficulty in integrating them with on-chip electronics [19]}.
Solid-state nanopores overcome these drawbacks, and also provide
some additional advantages like possibility of other types of detection, different
from ionic current [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, [26]}, [27]}, [28]}.
But one big problem remains, the synthetic membranes used for detection are more than
10 nm thick, which means that several nucleotides will occupy the nanopore at the same
time (distance between two consecutive nucleotides is 0.34 nm in double-stranded DNA
and 0.5 nm in ss-DNA [29]}, [30]}) and it becomes impossible to perform single-molecule
base-specific measurements. Graphene, a single layer allotrope of carbon [31]},
provides a new way to develop nanopore based technique for the single-molecule sequencing of DNA.
Graphene is much more superior over the conventional solid-state nanopores as the nanopore
dimension can be controlled to the atomic length scale as well as the thickness of the membrane
can be of the order of the distance between two consecutive nucleotides in a DNA chain.
Graphene shows better electromechanical properties [32]}. Graphene based nanopore
also offers several ways of sequential detection by means of transverse tunnelling [33]}, [34]},
nanoribbon conductance [35]}, [36]}, [37]} and also using mechanical deformation of
DNA [38]} followed by some experimental realizations [39]}, [40]}.
